Respiratory viruses are the number one cause of mortality in infants, however, little is known about the mechanisms that regulate airway antiviral immune responses in this vulnerable age group. MicroRNAs (miRNA), are small non-coding RNA molecules that regulate gene expression and immune responses. Notably, some miRNAs are selectively exported in extracellular vesicles (exosomes) by immune cells to induce cell-to-cell genetic reprogramming. Our preliminary studies in infants showed that miR-155, a potent enhancer of interferon (IFN)-mediated antiviral immunity, is the most abundant miRNA found in exosomes derived from nasal washes (n=20) during rhinovirus (RV) infection, the most common cause of viral respiratory illnesses in early life. This novel finding suggests that miR-155 plays a key role in the host defense against RV in humans and thus may be a new target for diagnostic and therapeutic applications. This proposal is focused on elucidating the biological effects of miR-155 in the nasal airway epithelium (AE), the primary target of RV infection in humans. Our central hypothesis is that exosomal miR-155 increases IFN responses in the nasal AE cells of infants and enhances their airway antiviral immunity during RV infection. Two Specific Aims are proposed: Specific Aim 1. Define the exosomal miR-155 response during in vivo RV infection in infants and determine its association with airway immune responses and clinical disease. Our hypothesis is that high nasal exosomal miR-155 levels provide stronger airway antiviral (Th1) responses, faster clearance of RV and decreased respiratory disease severity. Specific Aim 2. Establish the effects of exogenous miR-155 therapy in infant-derived nasal airway epithelial cells during in vitro RV infection. Our hypothesis is that exogenous miR- 155 treatment (synthetic miR-155 mimic loaded in liposomes) increases antiviral IFN production and inhibits RV replication in primary nasal AE cells from infants. This transdisciplinary proposal takes advantage of our access to a large population of infants and state-of-the-art resources for Genomics and Airway Biology. Given the unique nature of our samples (nasal epithelium from infants), the proposed molecular and genomic studies during RV infection in vivo (Aim1) and in vitro (Aim2) will greatly advance the field providing ground-breaking new insights into the potential mechanisms regulating early life airway immune responses against respiratory viruses in humans, a markedly understudied area.